[0001] The present invention relates to providing printed circuit boards (PCB's) with a
cover coat, and in particular, to a method for creating the cover coat pattern in
a photoresist overlayer simultaneously with the pattern for pads, lines etc. in the
copper foil of the board.
[0002] In the prior art, PCB's are manufactured in two basic steps. The first step is creating
the pattern for the conducting material, e.g. copper, and the second step is to cover
the PCB with a protective coating except on the contact pads.
[0003] In the first step a polyimide sheet substrate, e.g. of Kapton (Kapton is a registered
trademark of DuPont Company), is provided with a thin foil or layer of a conductive
material, such as copper. The copper is covered with a photoresist (e.g. AZ1375, which
is a trademark of Shipley) and a photo mask containing the circuit pattern is placed
on the resist. The photoresist is exposed to light through the mask, whereby the portions
that were exposed to light, are susceptible to removal by a suitable developer, whereas
the unexposed areas, defining the circuit pattern, remain. After exposure the mask
is removed, and the soluble areas are washed away. The exposed copper is etched away
leaving a circuit pattern. The photoresist is again exposed and developed, or removed
by a suitable solvent thus finishing the printed circuit panel.
[0004] In the second step the normal procedure is to blanket coat a printed circuit panel,
e.g. by silk screen printing, with a negative working photo-sensitive material (by
negative working is understood that illuminated parts are polymerized, whereas remaining
parts can be washed away; an example of such a material is cover coat 5500 which is
a trademark of Lea Ronal). A photo mask, with a pattern for those areas to be covered,
is aligned to the panel and exposed to light.
[0005] This alignment of the mask for the cover coat with those parts of the PCB that are
not to be covered, i.e. the pads, is very difficult, especially when there is a high
circuit density, and the operation is time consuming and thus very expensive.
[0006] Also known in prior art is the use of a special multi-density mask for the pattern
generation. Such a mask is disclosed in IBM Technical Disclosure Bulletin Vol. 19,
No. 12, May 1977, pages 4539 and 4540, and in Vol. 20, No. 3, August 1979, pages 964-5.
[0007] The described mask is such that opaque portions correspond to lands, partially transparent
portions correspond to lines and fully transparent portions correspond to background
substrate.
[0008] The present invention uses a modification of this mask, which will be described in
detail later.
[0009] In view of the above mentioned disadvantages with prior art techniques, it is the
object of the present invention to provide a method for cover coating printed circuit
boards wherein the alignment problem is eliminated.
[0010] This is achieved by the invention as claimed in the appended claims in that the patterns
for both the conductor lay-out and for the cover coat created simultaneously in one
single exposure of the photo-resist through the mark.
[0011] An essential feature for being able to carry out the invention is an unexpected interaction
between the photoresist and the cover coat upon exposure to light. It turns out that
provided the layers of the respective materials are thin enough, there occurs a light
induced reaction between the materials, rendering the exposed portions more soluble
in a specific solvent, than the non-exposed portions.
[0012] One way of carrying out the invention is described in detail below with reference
to the drawings, in which
Fig 1 is a schematic picture in top view of a mask for carrying out the invention,
and
Fig 2.1 - 2.6 show the sequence of steps for making a PCB provided with a cover coat
in accordance with the invention; the figures show cross-sections of a simplified
circuit board during manufacturing.
[0013] The tri-level mask shown schematically in figure 1 has opaque portions 1 for creating
contact pads, fully transparent portions 2 for background substrate and semi transparent
(10-30% transmission) areas 3 for lines. Its function will be described below.
[0014] In Figs. 2.1 - 2.7, one way of carrying out the method according to the invention
is shown as a sequence of steps.
[0015] Having prepared a suitably patterned mask Figure 1, a substrate comprising a base
material 11 (eg: Kapton) covered over at least one surface (generally the whole surface
is covered although this is not essential) with a thin foil of copper 12 and an overlayer
of a photoresist 13 (eg Shipley AZ1375), is exposed to light through the mask 14 as
shown is Figure 2.1. The photoresist is applied as a dispersion and dried before exposure.
[0016] Figure 2.2 shows the board after exposure and the selective wash away of those parts
that were fully exposed, ie: those areas constituting the background substrate (Kapton),
by using a mild developer (eg: AZ Developer).
[0017] The circuit is etched thus removing the exposed copper areas 15 to give the board
as shown in Figure 2.3.
[0018] Figure 2.4 shows the board after a more aggressive developer (eg: microposit 351
developer Shipley) has be used to remove the resist - 16 from the areas exposed to
the semi-transparent parts 3 of the mask 14, ie: the parts constituting the lines.
[0019] A cover coat in the form of a negative working photo sensistive material 17 such
as Lea Ronal 5500 (an epoxy modified acrylic) is now blanket screened onto the panel
as shown in Figure 2.5, dried and blanket exposed.
[0020] Exposure to light unexpectedly induces a modification of those parts - 18 of the
cover coat material that interface with the photoresist (Figure 2.6) such that the
modified material can be selectively dissolved along with the reaction products of
the positive photoresist to leave a cover coated panel with coated areas 19 and bare
pads 20 as shown in Figure 2.7.
[0021] The mechanism or the chemistry of this phenomenon is not fully understood, but is
thought to be inhibition of the photo cross linking of the cover coat material by
one or more components of the photo resist.
[0022] The cover coat is finally cured according to the manufacturers instructions.
[0023] Control of the time between applying and drying the cover coat allows some dissolution
of the photoresist without excessive loss of definition by smearing (about 5 minutes
is suitable).
[0024] The exposure time may be slightly increased to ensure complete scission of the positive
photoresist under the cover coat.
[0025] The final development of the cover coat is carried out an follows:
1. 2 minute soak in aqueous alkaline solution with a pH of 12-13 containing sodium
phosphates and silicates but no hydroxide .
2. Neutralize the alkali with 2-5% HCl.
3. Rinse in an organic solvent such as N-butyl-acetate or acetone.
[0026] Thus, by means of invention a simple and inexpensive method of providing a cover
coat on a PCB is disclosed. lt should be evident to the skilled man that the invention
can be varied within the scope of the appended claims.
1. Method of providing a printed circuit board with a cover coat on a selected area
thereof
characterised by the steps of:
i) applying a photoresist material to an area of the board not to be provided with
a covercoat;
ii) applying a photosensitive material to both the photoresist and the area of the
board to be provided with covercoat;
iii) exposing the board to light to promote reaction of photoresist material with
photosensitive material;
iv) removing the reaction product of photoresist material and photosensitive material;
and (v) curing the remaining photosensitive material to convert to covercoat.
2. Method according to claim 1 wherein the cover coat is provided in alignment with
the circuit pattern on the board.
3. Method according to Claim 2, including the preliminary steps of:
simultaneously creating a latent image of the circuit pattern and of the cover coat
respectively in a photoresist of the base material, by exposing the photoresist to
light through a mask, and
individually developing the respective patterns, whereby the alignment of the cover
coat on the circuit pattern is achieved.
4. Method according to Claim 3, wherein the mask is a multi-density mask defining
the patterns for both the cover coat and the circuit pattern.
5. Method according to Claim 4, wherein the pattern of the mask is such that areas
defining contact pads are opaque, areas defining circuit lines are semi-transparent,
and areas defining background are fully transparent.
6. Method according to any of the preceding claims, wherein the photosensitive material
is an epoxy modified acrylic and the photoresist is a formaldehyde novolak resin with
a diazo ketone sensitizer.
7. Method according to any of the preceding claims, wherein the photosensitive material
is allowed to dry for approximately 5 minutes after applying the same as an overlayer
on the board, and before blanket exposing it to light.
8. Cover coat for a PCB, made by the method according to any of claims 1 - 5, characterised
in that it is fully congruent with the circuit pattern except for the contact pads,
which are exposed for the purpose of soldering components thereon.
9. Cover coat according to Claim 8, wherein the congruency is achieved by using one
single photo mask and one single exposure for the manufacture of the entire PCB including
conductive pattern and the cover coat.